MECHANISM OF OLF ACTION. 661 



Among the experiments upon the higher orders of animals, in which the 

 olfactory nerves have been divided, may be cited, as open to no objections, 

 those of Vulpian and Philipaux, upon dogs. It is well known that the sense 

 of smell usually is very acute in these animals. Upon dividing or extirpating 

 the olfactory, bulbs, " after the animal had completely recovered, it was de- 

 prived of food for thirty-six or forty-eight hours ; then, in its absence, a piece 

 of cooked meat was concealed in a corner of the laboratory. Animals, suc- 

 cessfully operated upon, then taken into the laboratory, never found the bait ; 

 and nevertheless, care had been taken to select hunting-dogs." This experi- 

 ment is conclusive ; more so than those in which animals deprived of the 

 olfactory bulbs were shown to eat faeces without disgust, for this sometimes 

 occurs in dogs that have not been multilated. 



Comparative anatomy shows that the olfactory bulbs generally are devel- 

 oped in proportion to the acuteness of the sense of smell. Pathological facts 

 show, in the human subject, that impairment or loss of the olfactory sense is 

 coincident with injury or destruction of these ganglia. Cases have been 

 reported in which the sense of smell was lost or impaired from injury to the 

 olfactory nerves. In nearly all of the cases on record, the general sensibility 

 of the nostrils was not affected. 



Mechanism of Olfaction. Substances that have odorous properties give 

 off material emanations, which must come in contact with the olfactory mem- 

 brane before their peculiar odor is appreciated. This membrane is situated 

 high up in the nostrils, is peculiarly soft, is abundantly provided with glands, 

 by the secretions of which its surface is kept in proper condition, and it pre- 

 sents the peculiar nerve-terminations of the olfactory filaments. 



In experimenting upon the sense of smell it has been found difficult to 

 draw an exact line of distinction between impressions of general sensibility 

 and those which attack the special sense, or in other words, between irritating 

 and odorous emanations ; and the vapors of ammonia, acetic acid, nitric acid 

 etc., undoubtedly possess irritating properties which overpower their odorous 

 qualities. It is unnecessary in this connection to discuss the different varie- 

 ties of odors recognized by some of the earlier writers, as the fragrant, aro- 

 matic, fetid, nauseous etc., distinctions sufficiently evident from their mere 

 enumeration ; and it is plain enough that there are emanations, like those 

 from delicately scented flowers, which are easily recognizable by the sense of 

 smell, while they make no impression upon the ordinary sensory nerves. The 

 very marked individual differences in the delicacy of the olfactory organs in 

 the human subject and in different animals are evidence of this fact. Hunt- 

 ing-dogs recognize odors to which most persons are absolutely insensible ; 

 and certain races of men are said to possess a remarkable delicacy of the sense 

 of smell. Like the other special senses, olfaction may be cultivated by atten- 

 tion and practice, as is exemplified in the delicate discrimination of wines, 

 qualities of drugs etc., by experts. 



After what has been said concerning the situation of the true olfactory 

 membrane in the upper part of the nasal fossae and the necessity of particles 

 impinging upon this membrane in order that their odorous properties may 



